The invention relates to a method for controlling the hybrid drive of a motor vehicle, which hybrid drive comprises the following components: an internal combustion engine, an axle drive transmission, at least one electric machine, at least one clutch, at least one energy store, and at least one driven axle. In the simplest case, the hybrid drive to be controlled is composed of an internal combustion engine, an electric machine that is arranged in series and that can be operated as a motor and as a generator, a single-stage (that is to say non-shiftable) step-down transmission and differential, as an axle drive transmission, for a driven axle. The hybrid drive to be controlled may however also comprise a plurality of electric machines in a more complex arrangement, a plurality of clutches and a plurality of—in particular two—driven axles.
In known controllers, the driver demand and capacities of the torque sources and sinks (this means the electric machine and the internal combustion engine) are coordinated with one another in branched and complex logical decision trees, wherein the mode that leads to the fulfillment of the driver demand is selected at an early stage, and only then are the operating points determined and, as a result, the activation signals for the individual components generated. For this reason alone, it is not possible for the capacities of all the components to be optimally utilized at the same time and for said components to also be operated at their most economical operating points at which they are subjected to the least wear. In the case of a shift transmission, this is exacerbated by the fact that the transmission gear to be selected must also be incorporated in the optimization. With a branched logic of said type, an adaptation to driving dynamics requirements can only be obtained with difficulty. This is particularly disadvantageous if a limitation of wheel slip is desired or even, in a vehicle with two driven axles, the distribution of the torque between the two axles is to be controlled.
Here, and in the following text, the word “mode” is to be understood to mean the sum of drive constellations and operating states of the components, for example: “driving with drive provided by the internal combustion engine”, “driving with electric drive”, “driving with mixed drive”, “coasting”, “accelerating”, “charging the battery”, “braking with energy recuperation”, etc. If the system comprises a plurality of electric machines, a plurality of clutches and two driven axles, there are also several further modes. If said modes are each linked with possible transmission gears, an even greater number of modes are generated, which number may run into three figures.
A further problem with heavily branched logic which can only be overcome with difficulty is the transition from one mode to another. According to U.S. Pat. No. 6,321,143, it is sought to solve said problem by means of heirarchically arranged priorities between the individual modes, and a system of flags, the definition of which is very complicated, and the effect of which is doubtful.
WO 02/26520 A1 discloses a simpler type of control of a hybrid drive, in which a heirarchical separation is provided between the definition of the nominal operating state and the distribution of the torque demand between the provided torque sources and sinks. It is thereby possible to reduce the system complexity, albeit with restricted functionality. In said document, an “efficiency optimizing” mode is also provided, but said mode optimizes not the efficiency of the entire arrangement but rather only the efficiency of the internal combustion engine; the efficiencies and restrictions of the other components remain unconsidered.
It is therefore an object of the invention to create a control method and a controller for hybrid drives which, while avoiding the above-specified disadvantages, or by solving the above-stated problems, offer the greatest possible efficiency, the maximum service life of the components, a high level of driving comfort and the capability of allowing for all driving dynamics requirements. Driving comfort also includes a free and smooth transition from one mode to another.